The present invention relates to sampling oscilloscopes, and more particularly, to a method and apparatus for measuring the complex frequency response of the sampling circuit utilized in such oscilloscopes.
Many electrical signals of interest have bandwidths that are much greater than the bandwidth of a typical oscilloscope. Hence, measuring such signals on an oscilloscope presents a problem. A sampling oscilloscope circumvents this problem for repetitive signals by utilizing a sampling circuit that measures the signal over a very brief time interval and displaying the resulting sample as one point of a graph. Typically, one sample is taken during each period of the repetitive signal. The time of the sample relative to the beginning of the signal repetition is varied in each period. Hence, the collection of samples can be displayed to provide a conventional display of voltage as a function of time.
A key component in such oscilloscopes is the sampling circuit that captures the signal voltage over the short sampling time interval. The sampling circuit is basically a very fast switch coupled to a capacitor that stores the potential value seen when the switch opens. The impulse response of this switch determines the time value over which the signal is measured. Variations in signal intensity that occur over time intervals much smaller than this time cannot be easily observed. Hence, the impulse response of this switch must either be known or must be faster than some predetermined design specification. Unfortunately, measuring the impulse response of the sampling stage is difficult.
For one class of sampling switches, the impulse response function is virtually identical to a signal that is generated by the switch and which exits through the same connector as the signal that is normally to be measured enters. In this type of sampling oscilloscope, the impulse response function can be measured by using this signal as the input to an identical oscilloscope. In this type of xe2x80x9cnose-to-nosexe2x80x9d testing, a common trigger pulse triggers the two oscilloscopes and the internal time bases of the oscilloscopes are used to measure the convolution of the impulse response function with itself. The actual impulse response function can then be extracted from these measurements. This type of calibration procedure is discussed in detail in Jan Verspecht and Ken Rush, xe2x80x9cIndividual Characterization of Broadband Oscilloscopes with a Nose-to-Nose Calibration Procedurexe2x80x9d, IEEE Transactions on Instrumentation and Measurement, vol. 43, no. 2, April 1994, pp. 347-354.
While this prior art calibration procedure provides the desired data, the calibration process is very time consuming, often requiring more than 24 hours to complete due to the lengthy calibration processes needed. As a result of the time needed to calibrate the sampling circuit, the procedure is too expensive to be performed on each sampling unit during manufacture. In addition, re-calibration of the sampling circuit in the field is normally impossible.
The high time costs of performing this type of calibration are the result of a number of factors. First, the time bases in the oscilloscopes determine the sampling points. While small non-linearities in these time bases are acceptable in normal oscilloscope operation, such non-linearities introduce significant errors into the impulse response measurements. To remove these artifacts, the time bases must be very accurately calibrated. The time bases, however, drift with time and temperature by a significant amount that would introduce significant errors into the impulse response measurements. Hence, the time base calibration must be performed at the beginning and end of the impulse response measurement.
Second, the common trigger source and two time bases introduce a significant amount of xe2x80x9cjitterxe2x80x9d that must also be taken into account by the calibration software. To correct for this jitter, the measurements must be repeated a large number of times to obtain relevant averages. In addition, the statistical distribution of the jitter must also be measured.
Broadly, it is the object of the present invention to provide an improved apparatus and method for calibrating the sampling stage of a sampling oscilloscope.
This and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
An apparatus and method for calibrating a first sampling circuit with a second sampling circuit are constructed according to the embodiments of the invention. Each sampling circuit has a signal input for receiving a signal to be sampled and a trigger input for receiving a trigger pulse that opens a switching circuit in that sampling circuit. The second sampling circuit includes a sample and hold circuit that provides an output indicative of the potential at the sampling circuit input of the second sampling circuit at a time determined by a signal at the trigger pulse input of the second sampling circuit. The signal inputs of the first and second sampling circuits are connected by a signal conductor. The calibration apparatus includes a trigger pulse generating circuit for generating a sequence of trigger pulse pairs, each trigger pulse pair including a first pulse that is delayed relative to a second pulse. A connecting circuit applies the first pulse to the trigger input of the first sampling circuit and the second pulse to the trigger input of the second sampling circuit. A controller sets the delay between the first and second pulses and measures the output of the sample and hold circuit in the second sampling circuit for each delay. The trigger pulse generating circuit preferably includes a variable delay line that is controlled by an actuator such as a stepping motor. The second sampling circuit is preferably part of a sampling module in an oscilloscope. The module is connected to the oscilloscope by an interface connector and the connecting circuit of the present invention. The connecting circuit of the present invention includes an extension card having a first connector that mates to the interface connector and a second connector that mates to the module. The second sampling circuit is assumed to have an impulse response that is approximately the same as that of the first sampling circuit. In the event that the assumption of equivalence of the first and second sampling circuits cannot be made, a similar procedure using three non-equivalent sampling circuits in pairs can be utilized.